1. Field of the Invention
The present invention relates to a superconducting coil, and more specifically, it relates to an oxide high-temperature superconducting coil particularly employable under a relatively high temperature, which can provide a high magnetic field with small power and is applicable to magnetic separation or crystal pulling.
2. Description of the Prior Art
A coil prepared by winding a normal conductor such as copper or a metal superconductor exhibiting superconduction at the liquid helium temperature has been generally employed.
In case of providing a high magnetic field with a coil prepared by winding a copper wire, however, it is necessary to cool the coil, remarkably generating heat, by forcibly feeding water or the like. Therefore, the coil prepared by winding a normal conductor disadvantageously requires high power consumption, and is inferior in compactness and hard to maintain.
On the other hand, the coil prepared by winding a metal superconductor must be cooled to a cryogenic temperature of about 4 K, to disadvantageously result in a high cooling cost. In addition, the coil which is employed under such a cryogenic temperature with small specific heat is so inferior in stability that the same readily causes quenching.
It has been proved that an oxide high-temperature superconducting coil which is employable under a relatively high temperature as compared with the metal superconducting coil allows employment in a region with high specific heat and is remarkably excellent in stability. Thus, the oxide high-temperature superconducting coil is expected as a material for a superconducting magnet which is easy to use.
An oxide high-temperature superconducting wire, which exhibits superconduction at the liquid nitrogen temperature, is relatively inferior in critical current density and magnetic field property at the liquid nitrogen temperature. Under the present circumstances, therefore, the oxide high-temperature superconducting coil is employed as a coil for providing a low magnetic field at the liquid nitrogen temperature.
While the oxide high-temperature superconducting coil is employable as a coil of higher performance at a temperature lower than the liquid nitrogen temperature, liquid helium is too costly and intractable for serving as a practical coolant. To this end, an attempt has been made to cool the oxide high-temperature superconducting coil to a cryogenic temperature with a refrigerator which is at a low operating cost and tractable.
In general, a dip-cooled metal superconducting coil is operated with a current which is considerably smaller than the critical current to be employed in a state hardly generating heat, in order to prevent quenching. Alternatively, a coolant is forcibly fed into the superconducting wire, or the superconducting coil is cooled while defining clearances between turns of the superconducting wire for allowing sufficient passage of the coolant.
On the other hand, a recent conduction-cooled superconducting coil is conduction-cooled from around the same, to be employed in a state hardly generating heat.
The oxide high-temperature superconducting coil can be cooled by a method similar to that for the metal superconducting coil. However, an oxide high-temperature superconducting wire, which has a high critical temperature and is highly stable due to loose normal conductivity transition, is hard to quench. Therefore, the oxide high-temperature superconducting coil is expected to be operated with a high current up to a level close to the critical current. In order to operate the superconducting coil with such a current up to a level close to the critical current, it is necessary to sufficiently cool the superconducting coil. Particularly in conduction cooling with a refrigerator, it is necessary to cool the superconducting coil without increasing its temperature by small heat generation.
However, it is difficult to efficiently conduction-cool the superconducting coil with a refrigerator, due to limitation in cooling ability and cooling path.
In the conventional method, conduction cooling is performed only from around the superconducting coil. While the turns of the superconducting wire are electrically isolated from each other in the superconducting coil, the material employed for such isolation is extremely inferior in heat conduction. In conduction cooling from around the coil, therefore, it is difficult to cool the coil up to its interior with low heat resistance. If small heat generation takes place in the interior of the coil, the temperature of the coil is extremely increased. In the conventional cooling method, therefore, heat generation allowed to the coil is extremely small, and the operating current for the coil is considerably smaller than the critical current.
The oxide high-temperature superconducting coil is expected to be operated with a current closer to the critical current, due to high stability of the oxide high-temperature superconducting wire. Further, the oxide high-temperature superconducting coil tends to gradually generate heat when operated with a current smaller than the critical current, due to a small n value (the way of rise of current-voltage characteristics). In order to operate the oxide high-temperature superconducting coil, therefore, it is necessary to more efficiently cool the coil as compared with the prior art.
The n value is employed in the following relational expression: ##EQU1##
An oxide superconductor has magnetic field anisotropy. A superconducting wire shaped to orient such an oxide superconductor exhibits magnetic field anisotropy, is intolerant of a magnetic field which is parallel to its C-axis, and causes further reduction of the critical current density. When the oxide superconductor is shaped in the form of a tape, the C-axis is generally oriented perpendicularly to the tape surface.
Japanese Patent Laying-Open No. 8-316022 (1996) discloses a structure of a superconducting coil suppressing frictional heat between turns of an insulated conductor for improving cooling performance between a superconducting wire and a refrigerator. This gazette discloses a superconducting coil which is obtained by coating a superconducting wire, forming a prescribed material when heat-treated at a temperature exceeding 400.degree. C., with an inorganic or mineralized insulator layer for preparing an insulated conductor, winding the insulated conductor for forming a wire part and thereafter heat-treating the same. When the insulated conductor is wound, a fixative of aluminum or an aluminum alloy which is softened or melted at the heat treatment temperature is wound into the wire part. This superconducting coil is prepared by the so-called wind-and-react method (a method of forming a superconductor by reaction heat treatment after winding a coil).
However, this superconducting coil has the following problems: First, the superconducting coil must be heat-treated at a temperature exceeding 400.degree. C. Thus, the material for the insulator layer is limited, to result in a smaller degree of freedom. In general, the material for the insulator layer has a large thickness. Consequently, the ratio of the wire forming the superconducting coil is reduced, to deteriorate the performance of the superconducting coil.
Further, the aforementioned superconducting coil must be heat-treated in inert gas or reducing gas. If the superconducting coil is heat-treated in an oxygen atmosphere, aluminum or the aluminum alloy employed as the fixative is oxidized, to deteriorate heat conductivity. When a superconducting wire consisting of an oxide high-temperature superconductor is employed and heat-treated in inert gas or reducing gas, superconduction properties such as the critical temperature, the critical current density and the like are deteriorated.
In the structure of the aforementioned superconducting coil, further, the fixative is thermally connected to the superconducting wire through the insulator layer, which is inferior in heat conductivity to a metal. Thus, the cooling property is deteriorated.